A RemoteApp source is an individual Remote Desktop Session Host server or a farm of identically configured Remote Desktop ...

A RemoteApp source is an individual Remote Desktop Session Host server or a farm of identically configured Remote Desktop Session Host servers. When you add a RemoteApp source, the RemoteApp programs from that RemoteApp source will be available through RemoteApp and Desktop Connection. You can add multiple RemoteApp sources.

For more information, see Configuring a RemoteApp Source.